Fix child and child_count methods to use resolved children

Author: nicoburns

src/compute/taffy_tree.rs

@@ -307,9 +307,9 @@ fn round_layout(tree: &mut Taffy, node_id: NodeId, cumulative_x: f32, cumulative
     layout.size.width = round(cumulative_x + unrounded_layout.size.width) - round(cumulative_x);
     layout.size.height = round(cumulative_y + unrounded_layout.size.height) - round(cumulative_y);
 
-    let child_count = tree.child_count(node_id).unwrap();
+    let child_count = tree.children[node_id.into()].len();
     for index in 0..child_count {
-        let child = tree.child(node_id, index);
+        let child = tree.children[node_id.into()][index];
         round_layout(tree, child, cumulative_x, cumulative_y);
     }
 }

src/tree/taffy_tree/tree.rs

@@ -108,11 +108,10 @@ fn find_children_recursive(tree: &Taffy, node: NodeId, out: &mut Vec<NodeId>) {
     }
 }
 
-impl LayoutTree for Taffy {
-    type ChildIter<'a> = RefCellVecIter<'a>;
-
-    #[inline(always)]
-    fn children(&self, node: NodeId) -> Self::ChildIter<'_> {
+impl Taffy {
+    /// Returns the resolved children, taking into account `Display::Contents`
+    /// Will use cached result if available, else compute and cache.
+    fn resolve_children(&self, node: NodeId) -> RefMut<'_, Vec<NodeId>> {
         let mut cache = self.node_children_cache.borrow_mut();
 
         // If the cache key does not match the requested node_id, then recompute the children for
@@ -124,12 +123,25 @@ impl LayoutTree for Taffy {
         }
 
         // In all cases, return a reference into the cache
-        RefCellVecIter { children: RefMut::map(cache, |c| &mut c.children), index: 0 }
+        RefMut::map(cache, |c| &mut c.children)
+    }
+}
+
+impl LayoutTree for Taffy {
+    type ChildIter<'a> = RefCellVecIter<'a>;
+
+    #[inline(always)]
+    fn children(&self, node: NodeId) -> Self::ChildIter<'_> {
+        RefCellVecIter { children: self.resolve_children(node), index: 0 }
     }
 
     #[inline(always)]
     fn child_count(&self, node: NodeId) -> usize {
-        self.children[node.into()].len()
+        self.resolve_children(node).len()
+    }
+    #[inline(always)]
+    fn child(&self, node: NodeId, id: usize) -> NodeId {
+        self.resolve_children(node)[id]
     }
 
     #[inline(always)]
@@ -155,11 +167,6 @@ impl LayoutTree for Taffy {
         }
     }
 
-    #[inline(always)]
-    fn child(&self, node: NodeId, id: usize) -> NodeId {
-        self.children[node.into()][id]
-    }
-
     #[inline(always)]
     fn measure_child_size(
         &mut self,